📖 add note ClusterClass compatibility info to validating webhooks section

[chrischdi]

What this PR does / why we need it:
Follow up to #6710

Follow-up documentation via discussion at #6710 (comment)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

/cherry-pick release-1.2

[killianmuldoon]

Should this also go into the 1.1 -> 1.2 migration notes.

Just checked, it's already there 😆

[sbueringer]

/cherry-pick release-1.2

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.2 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.2

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[sbueringer]

Comment looks good for the provider guide. Not sure if/what we want to write something in the contract as well. I'll defer to @fabriziopandini

[fabriziopandini]

/lgtm
/approve

[k8s-ci-robot]

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[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #6909

In response to this:

/cherry-pick release-1.2

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.